1. Field of the Invention
This invention relates to a radiation image recording/read-out method, a radiation image recording/read-out apparatus and a stimulable phosphor sheet, and more particularly to a radiation image recording/read-out method and a radiation image recording/read-out apparatus in which a radiation image is recorded from one side of a stimulable phosphor sheet and the radiation image recorded on the stimulable phosphor sheet is read out from the other side of the stimulable phosphor sheet, and a stimulable phosphor sheet suitable for the method and the apparatus.
2. Description of the Related Art
When certain kinds of phosphor are exposed to radiation such as X-rays, xcex1-rays, xcex2-rays, electron-beams, ultraviolet rays and the like, they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as xe2x80x9ca stimulable phosphorxe2x80x9d. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as xe2x80x9cstimulated emissionxe2x80x9d. There has been known a radiation image recording and reproducing system in which a stimulating light beam such as a laser beam is projected onto a stimulable phosphor sheet (a sheet provided with a layer of the stimulable phosphor) which has been exposed to radiation passing through an object such as a human body to have a radiation image of the object stored on the stimulable phosphor sheet, and the stimulated emission emitted from the stimulable phosphor sheet is photoelectrically detected, thereby obtaining an image signal (a radiation image signal). A radiation image of the object is reproduced as a visible image on the basis of the radiation image signal on a recording medium such as a photographic film or a display such as a CRT. See, for instance, Japanese Unexamined Patent Publications Nos. 55(1980)-12429, 56(1981)-11395 and 56(1981)-11397.
The stimulable phosphor sheet employed in such radiation image recording and read-out generally has a layered structure comprising a stimulable phosphor layer and a support layer which supports the stimulable phosphor layer. Projection of the radiation, projection of the stimulating light beam and detection of the stimulated emission are generally performed from the stimulable phosphor layer side. There have been known various apparatuses for recording on and/or read-out from the stimulable phosphor sheet, e.g., a radiation image recording apparatus which records radiation image information on the stimulable phosphor sheet, a radiation image read-out apparatus which reads out radiation image information recorded on the stimulable phosphor sheet, and a radiation image recording/read-out apparatus which is formed by integrating a radiation image recording apparatus and a radiation image read-out apparatus in a unit and includes a built-in stimulable phosphor sheet (will be referred to as xe2x80x9ca built-in radiation image recording/read-out apparatusxe2x80x9d, hereinbelow).
In the built-in radiation image recording/read-out apparatus, there has been a demand that for miniaturizing the apparatus by arranging the apparatus so that a latent image of the object is recorded by projecting a radiation from the support layer side and the latent image is read out by projecting the stimulating light beam onto the stimulable phosphor sheet from the stimulable phosphor layer side and detecting the stimulated emission from the stimulable phosphor layer side.
That is, in the system where recording of the image and read-out of the image are performed from the same side of the stimulable phosphor sheet (this system will be referred to as xe2x80x9cthe same-side recording/read-out systemxe2x80x9d, hereinbelow), there is required a conveyor system for conveying the stimulable phosphor sheet to the read-out system and turning the stimulable phosphor sheet over to oppose the same side of the stimulable phosphor sheet to the read-out system after recording of the image, which increases the overall size of the apparatus. To the contrast, in the system where recording of the radiation image and read-out of the image are performed from opposite sides of the stimulable phosphor sheet (this system will be referred to as xe2x80x9cthe opposite-side recording/read-out systemxe2x80x9d, hereinbelow), the radiation image can be read out from the stimulable phosphor sheet without moving the stimulable phosphor sheet (or by moving by a very small distance) or turning over the same, which is advantageous to reduce the overall size of the apparatus.
However, in the opposite-side recording/read-out system, there has been a problem that the image quality of the radiation image obtained is bad as compared with that obtained by the same-side recording/read-out system.
That is, when the radiation is projected onto the stimulable phosphor sheet through the support layer, the latent image is formed in a part of the stimulable phosphor layer nearer to the support layer, and accordingly, when the stimulating light is projected onto the stimulable phosphor layer from the stimulable phosphor layer side, scattering and absorption of the stimulating light occur in the stimulable phosphor layer before the stimulating light reaches the part of the stimulable phosphor layer in which the latent image is formed, whereby the stimulating light is diverged and weakened. At the same time, the stimulated emission emitted from the part of the stimulable phosphor layer in which the latent image is formed is scattered and absorbed by the stimulable phosphor layer before it is radiated from the stimulable phosphor layer, whereby the area from which the stimulated emission is assumed to be emitted is broadened and the stimulated emission as radiated from the stimulable phosphor sheet is weakened. As a result, since noise is generated in the detected stimulated emission by scattered light (e.g., scattered stimulated emission, scattered stimulated emission emitted from surrounding stimulable phosphor stimulated by scattered stimulating light) and the stimulated emission is weakened, the image quality of the radiation image obtained by the opposite-side recording/read-out system is bad as compared with that obtained by the same-side recording/read-out system.
The stimulable phosphor sheets which have been conventionally employed are structured so that the radiation is projected from the stimulable phosphor layer side, and the power density of the stimulating light projected onto the stimulable phosphor sheet is generally about 4.5J/m2.
The image quality of the radiation image is generally evaluated on the basis of the sharpness (MTF) and the grainness (as the amount of stimulated emission increases, the grainness is improved and as the stimulated emission emitting area is widened, the sharpness of the image deteriorates), and the higher the sharpness (MTF) or the grainness is, the better, the image quality is evaluated to be.
In view of the foregoing observations and description, the primary object of the present invention is to provide a radiation image recording/read-out method and a radiation image recording/read-out apparatus which makes it feasible to reduce the overall size of the apparatus without deteriorating image quality of the radiation image obtained, and to provide a stimulable phosphor sheet suitable for the method and the apparatus.
In accordance with a first aspect of the present invention, there is provided a radiation image recording/read-out method comprising the steps of projecting radiation onto a stimulable phosphor sheet, having a reflective layer which is not longer than 5 xcexcm in scattering length and is interposed between a stimulable phosphor layer and a support layer, from the support layer side of the stimulable phosphor sheet, projecting stimulating light onto the stimulable phosphor sheet, which has been exposed to the radiation, from the stimulable phosphor layer side of the stimulable phosphor sheet, and detecting stimulated emission, emitted from the stimulable phosphor layer upon exposure to the stimulating light, from the stimulable phosphor layer side of the stimulable phosphor sheet.
In accordance with a second aspect of the present invention, there is provided a radiation image recording/read-out apparatus comprising a stimulable phosphor sheet having a reflective layer which is not longer than 5 xcexcm in scattering length and is interposed between a stimulable phosphor layer and a support layer, a radiation projecting means which projects radiation onto the stimulable phosphor sheet from the support layer side of the stimulable phosphor sheet, and a read-out means which projects stimulating light onto the stimulable phosphor sheet, which has been exposed to the radiation, from the stimulable phosphor layer side of the stimulable phosphor sheet and detects stimulated emission, emitted from the stimulable phosphor layer upon exposure to the stimulating light, from the stimulable phosphor layer side of the stimulable phosphor sheet.
It is preferred that the read-out means projects the stimulating light at a power density of not lower than 10J/m2.
It is preferred that the transmittance to the radiation of the part of the stimulable phosphor sheet between the part where the radiation enters the stimulable phosphor sheet and the part where the radiation impinges upon the stimulable phosphor layer be not smaller than 80%.
It is preferred that the stimulable phosphor layer be colored by a coloring agent which selectively absorbs light of a wavelength which is equal to that of the stimulating light.
The radiation image recording/read-out apparatus of the present invention need not be of a built-in type where a radiation image information recording system and a radiation image information read-out system are integrated into a unit, but may be of a separate type where a radiation image recording apparatus and a radiation image read-out apparatus which are separately provided are used in combination with each other.
In accordance with a third aspect of the present invention, there is provided a stimulable phosphor sheet comprising a reflective layer which is not longer than 5 xcexcm in scattering length and is interposed between a stimulable phosphor layer and a support layer, wherein radiation is projected onto the stimulable phosphor sheet from the support layer side, whereas stimulating light is projected onto the stimulable phosphor sheet from the stimulable phosphor layer side and stimulated emission, emitted from the stimulable phosphor layer upon exposure to the stimulating light, is detected from the stimulable phosphor layer side of the stimulable phosphor sheet.
The termxe2x80x9cscattering lengthxe2x80x9d as used here is the scattering length to the stimulating light or light of a wavelength which is equal to that of the stimulating light and means a mean distance by which the stimulating light travels straight before the stimulating light is once scattered. That the scattering length is shorter means that the light scattering performance is higher. The scattering length can be calculated by calculation based on Kubelka-Munk""s theory.
In the present invention, stimulating light which enters the stimulable phosphor sheet from the stimulable phosphor layer side and travels toward the support layer through the stimulable phosphor layer is reflected by the reflective layer and reenters the stimulable phosphor layer to stimulate the stimulable phosphor, whereby the amount of stimulated emission emitted from the stimulable phosphor layer increases. At the same time, since the component of the stimulated emission traveling toward the support layer is reflected by the reflective layer to be radiated outward from the stimulable phosphor sheet, the amount of detected stimulated emission further increases. Further, since the latent image of the object is formed in a part of the stimulable phosphor layer nearer to the support layer and the scattering length of the reflective layer is not longer than 5 xcexcm and is sufficiently short, the area over which the stimulating light is reflected by the reflective layer and the area over which the stimulated emission reenters the stimulable phosphor layer are limited to that near the optical path of the stimulating light entering from the stimulable phosphor layer side.
As a result, deterioration of the image quality (grainness and sharpness) of the radiation image obtained can be prevented even if the aforesaid opposite-side recording/read-out system, which is advantageous in reducing the size of the radiation image recording/read-out apparatus, is employed.
When the power density of the stimulating light is not lower than 10J/m2, the amount of stimulated emission emitted from a unit area of the stimulable phosphor layer can be increased with the stimulated emission emitting area suppressed from being widened, whereby the grainness of the radiation image obtained can be improved with deterioration of the sharpness suppressed and the image quality of the radiation image obtained can be further improved.
When the transmittance to the radiation of the part of the stimulable phosphor sheet between the part where the radiation enters the stimulable phosphor sheet and the part where the radiation impinges upon the stimulable phosphor layer is not smaller than 80%, deterioration of radiation energy stored in the stimulable phosphor can be prevented and the amount of stimulated emission emitted from the stimulable phosphor can be increased with the stimulated emission emitting area kept unchanged. As a result, the grainness of the radiation image obtained can be improved with the sharpness of the radiation image obtained kept unchanged and the image quality of the radiation image obtained can be further improved.
When the stimulable phosphor layer is colored by a coloring agent (e.g., ultramarine) which selectively absorbs light of a wavelength which is equal to that of the stimulating light, scattering of stimulated emission to be detected is suppressed, whereby the stimulated emission emitting area can be narrowed and the quality of the radiation image obtained can be changed toward a direction in which the grainness is roughened and the sharpness is enhanced.